1. Field of the Invention
The subject invention relates to a process for purifying polytetramethylene ether glycol or the corresponding diester prepared by the cationic polymerization of tetrahydrofuran. These products are purified by treating them with hydrogen in the presence of a hydrogenation catalyst.
2. Description of the Prior Art
The polymerization of tetrahydrofuran (THF) with the aid of oxonium ions as catalysts has become generally known by the basic work conducted by H. Meerwein and his associates. See, for example, Applied Chemistry, vol. 72, page 927, (1960).
Used as catalysts are either premanufactured compounds or those which are produced in situ. The following premanufactured trialkyloxonium salts are useful as catalysts:
[(C.sub.2 H.sub.5).sub.3 O] SbCl.sub.6 ; [(C.sub.2 H.sub.5).sub.3 O] BF.sub.4 ; [(C.sub.2 H.sub.5).sub.3 O] FeCl.sub.4 ; and [(C.sub.2 H.sub.5).sub.3 O] AlCl.sub.4.
In order to prepare such catalysts, Lewis acids such as BF.sub.3, AlCl.sub.3, SnCl.sub.4, SbCl.sub.4, SbCl.sub.5, FeCl.sub.3 or BF.sub.3 are reacted with ethylene oxide, epichlorohydrin, ortho-esters and acetals, .alpha.-chloroether, benzylchloride, triphenylmethylchloride, acetylchloride or .beta.-propiolactone as well as inorganic hydrogen acids such as HClO.sub.4, HBF.sub.4, HSO.sub.3 F, HSO.sub.3 Cl, H.sub.2 SnCl.sub.6, HIO.sub.3, HSbCl.sub.6 or HFeCl.sub.4 which can be used together with carboxylic acid anhydrides, carboxylic acid chlorides, SOCl.sub.2, POCl.sub.3 and benzenesulfonic acid chloride as well as aluminum alkyl compounds in combination with epichlorohydrin or water.
This list, which does not claim to be complete, describes the most important systems known to date and shows that there is an immense multitude of catalysts with which polyetramethylene ether glycols (PTHF) or the corresponding diesters can be produced. However, only a few catalysts have achieved industrial importance since only they permit the production of polymers which satisfy the market requirements as far as chemical and physical properties are concerned. A drawback of these catalysts is that they generally result in a polymer having a tendency to turn yellow or brown. The discoloration of the polymers becomes deeper as the temperature, at which the tetrahydrofuran was polymerized, increases.
Technical grade tetrahydrofuran contains slight quantities of impurities in a concentration of 10 to 500 ppm. The chemical nature of these impurities is not completely known. Although this technical grade tetrahydrofuran is basically of very high purity (it normally has a degree of purity of &gt;99.9 percent) these trace impurities evidently cause the above-referenced discoloration during polymerization. In addition to this, these impurities affect the reactivity during the preparation of polyesters of polyurethanes from the polytetramethylene ether glycol. Since color and reproducible processing are important characteristics for a polymer intended for industrial application, there is a need to eliminate the problems resulting from the impurities.
U.S. Pat. Nos. 3,980,672, 3,935,252 and 2,751,419 as well as German Published application No. 2,801,792 (U.S. Pat. No. 4,189,566) describe treatment methods for transforming technical grade tetrahydrofuran into a polymerizable monomer by pretreatment with zeolithic molecular sieves, strong mineral acids, organic sulfonic acids, silica gel or bleaching earths. Whereas the treatment described in the U.S. applications primarily causes the resultant polymers to have a residual acid number which does not require removal, the treatment with bleaching earths described in German Published application No. 2,801,792 results in the production of polymers with improved color number. It has been found, however, that these treatment methods cannot be used in a manner which is reproducible in every case and cannot be used with every technical grade tetrahydrofuran obtainable in the marketplace. The processes according to the above-referenced U.S. Patents further have the drawback that unreacted tetrahydrofuran must be carefully dried and purified prior to reuse since it originates from the wet processing of the polymer.
Evidently tetrahydrofuran contains different impurities. Since the impurities in most cases are not accurately identified, it cannot be foretold in every case by analytical methods whether the quality of the tetrahydrofuran is suitable for the polymerization. Therefore, and in spite of the pretreatment of the monomer, polymers which do not meet specifications are produced repeatedly.
There is a pressing need for developing a treatment method for polytetramethylene ether glycols or their diesters which facilitates removal of all impurities occurring in the polymer which are detrimental to further processing.